A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane (eardrum) 102, which moves the bones of the middle ear 103 (malleus, incus, and stapes) that vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns. It includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The cochlea 104 forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to generate electric pulses which are transmitted to the cochlear nerve 113, and ultimately to the brain.
Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104. To improve impaired hearing, auditory prostheses have been developed. For example, when the impairment is related to operation of the middle ear 103, a conventional hearing aid may be used to provide acoustic-mechanical stimulation to the auditory system in the form of amplified sound. Or when the impairment is associated with the cochlea 104, a cochlear implant with an implanted stimulation electrode can electrically stimulate auditory nerve tissue with small currents delivered by multiple electrode contacts distributed along the electrode.
FIG. 1 also shows some components of a typical cochlear implant system which includes an external microphone that provides an audio signal input to an external signal processor 111 where various signal processing schemes can be implemented. The processed signal is then converted into a digital data format, such as a sequence of data frames, for transmission into the implant 108. Besides receiving the processed audio information, the implant 108 also performs additional signal processing such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through an electrode lead 109 to an implanted electrode array 110. Typically, this electrode array 110 includes multiple electrodes on its surface that provide selective stimulation of the cochlea 104.
Cochlear implant systems employ stimulation strategies that provide high-rate pulsatile stimuli in multi-channel electrode arrays. One specific example is the “Continuous Interleaved Sampling (CIS)”-strategy, as described by Wilson et al., Better Speech Recognition With Cochlear Implants, Nature, vol. 352:236-238 (1991), which is incorporated herein by reference. For CIS, symmetrical biphasic current pulses are used, which are strictly non-overlapping in time. The rate per channel typically is higher than 800 pulses/sec. Other stimulation strategies may be based on simultaneous activation of electrode currents. These approaches have proven to be successful in giving high levels of speech recognition.
For an audio prosthesis such as a cochlear implant to work correctly, some patient-specific operating parameters need to be determined in a fit adjustment procedure where the type and number of operating parameters are device dependent and stimulation strategy dependent. Possible patient-specific operating parameters for a cochlear implant include:
THR1 (lower detection threshold of stimulation amplitude) for Electrode 1
MCL1 (maximum comfortable loudness) for Electrode 1
Phase Duration for Electrode 1
THR2 for Electrode 2
MCL2 for Electrode 2
Phase Duration for Electrode 2
. . .
Pulse Rate
Number of fine structure channels
Compression
Parameters of frequency→electrode mapping
Parameters describing the electrical field distribution
Some fit adjustment procedures allow applying the changes to the operating parameter without notable delay (“live-voice”); e.g. Willeboer and Smoorenburg, Comparing Cochlear Implant Users' Speech Performance With Processor Fittings Based On Conventionally Determined T And C Levels Or On Compound Action Potential Thresholds And Live-Voice Speech In A Prospective Balanced Crossover Study; Ear Hear, 27(6):789-798, December 2006; incorporated herein by reference.
Other types of audio prostheses have similar such parameters that need to be fit to the patient. For example, besides cochlear implant systems as such, some subjects with some residual hearing (partial deafness) are now benefiting from combined electric and acoustic stimulation (EAS) such as was first described in von Ilberg et al., Electric-Acoustic Stimulation Of The Auditory System, ORL 61:334-340 (1999), which is incorporated herein by reference. EAS systems combine the use of a conventional hearing aid (HA) device to provide acoustic-mechanical stimulation of lower audio frequencies to the subject's ear drum and a cochlear implant (CI) to provide intracochlear electrical stimulation of higher audio frequencies to the auditory nerve. For example, see Lorens et al., Outcomes Of Treatment Of Partial Deafness With Cochlear Implantation: A DUET Study, Laryngoscope, 2008 February: 118(2):288-94, which is incorporated herein by reference.
One common method for fit adjustment is to behaviorally find the threshold (THR) and maximum comfortable loudness (MCL) value for each separate stimulation electrode. See for example, Rätz, Fitting Guide for First Fitting with MAESTRO 2.0, MED-EL, Fürstenweg 77a, 6020 Innsbruck, 1.0 Edition, 2007. AW 5420 Rev. 1.0 (English_EU); incorporated herein by reference. Other alternatives/extensions are sometimes used with a reduced set of operating parameters; e.g. as suggested by Smoorenburg, Cochlear Implant Ear Marks, University Medical Centre Utrecht, 2006; U.S. Patent Application 20060235332; which are incorporated herein by reference. In normal operation, only stimulation amplitudes between MCL and THL for each channel are used. The dynamic range between MCL and THL typically is between 6-12 dB. However, the absolute positions of MCLs and THLs vary considerably between patients, and differences can reach up to 40 dB. To cover these absolute variations, the overall dynamic range for stimulation in currently used implants typically is about 60 dB.
There are several methods of setting fitting parameters like MCLs and THLs. For example, they can be estimated during the fit adjustment procedure by applying stimulation pulses and asking the patient about his/her subjective impression. Other more subjective fit adjustment procedures are also used, such as those which determine the syllable/word/sentence recognition score of users (e.g. “Freiburger Sprachtests”, “Göttinger Satztest”, “Einsilber-Reimtest nach von Wallenberg and Kollmeier”). The outcome of these tests can be used to monitor the performance of a user over time and to compare different settings of the fitting. In general, these tests are time consuming and a whole test session is required for each “setup” of the fitting.
These methods usually work without problems with postlingually deaf patients. However, problems can occur with prelingually or congenitally deaf patients, and in this group all ages—from small children to adults—are concerned. These patients are usually neither able to interpret nor to describe hearing impressions, and only rough estimations of fitting parameters like MCLs and THLs based on behavioral methods are possible. Especially the situation of congenitally deaf small children needs to be mentioned here. An adequate acoustic input is extremely important for the infant's speech and hearing development, and this input in many cases can be provided with a properly fitted cochlear implant. Moreover, the fitting procedure can be very time consuming and difficult, especially for children. Sometimes objective measurements are used to assist in the fitting procedure. These include:
nerve responses to electrical stimulation
brainstem responses
electrically evoked stapedius reflexes
Sometimes these methods are combined. However, these objective measures do not determine the user experience/performance and do not optimize the fitting regarding user experience/performance.
One approach for an objective measurement of MCLs and THLs is based on the measurement of the EAPs (Electrically Evoked Action Potentials), as described by Gantz et al., Intraoperative Measures of Electrically Evoked Auditory Nerve Compound Action Potentials, American Journal of Otology 15 (2):137-144 (1994), which is incorporated herein by reference. In this approach, a recording electrode in the scala tympani of the inner ear is used. The overall response of the auditory nerve to an electrical stimulus is measured very close to the position of the nerve excitation. This neural response is caused by the super-position of single neural responses at the outside of the axon membranes. The amplitude of the EAP at the measurement position is between 10 μV and 1800 μV. Information about MCL and THL at a particular electrode position can first of all be expected from the so called “amplitude growth function,” as described by Brown et al., Electrically Evoked Whole Nerve Action Potentials In Ineraid Cochlear Implant Users: Responses To Different Stimulating Electrode Configurations And Comparison To Psychophysical Responses, Journal of Speech and Hearing Research, vol. 39:453-467 (June 1996), which is incorporated herein by reference. This function is the relation between the amplitude of the stimulation pulse and the peak-to-peak voltage of the EAP.
Another interesting relation is the so called “recovery function” in which stimulation is achieved with two pulses with varying interpulse intervals. The recovery function as the relation of the amplitude of the second EAP and the interpulse interval allows conclusions to be drawn about the refractory properties and particular properties concerning the time resolution of the auditory nerve.
One problem of existing fit adjustment procedures is that they either don't optimize the fitting at all, or at most just for speech perception. And the subjective judgment of the patients is not used as it could or should be.